Over the years, numerous attempts have been made to improve the mechanical properties and fabricability of copper wire used in electric current carrying applications. In some of these applications, the mechanical properties of copper wire are as important as the electrical properties. Mechanical properties which are generally of concern to users of copper wire include ultimate tensile strength, elongation and elastic limit. To obtain the optimum mechanical behavior in the wire, it is desired to maximize ultimate tensile strength and elongation, and to minimize elastic ratio. These mechanical properties are important because there are many uses of copper wire which require it to be dereeled from a supply source at high speeds, pulled through a series of damaging tension devices, wound and formed onto rectangular or round bobbins, forcefully manipulated around sharp rectangular terminals, and joined with any of a number of different methods.
Many of the attempts which have been made to improve the strength and fabricability of copper wire have utilized a combination of mechanical working and heat treatment methods. U.S. Pat. No. 4,280,857 describes one such combination wherein wire is continuously directed through a plurality of drawing dies, then through a heater for an in-process anneal, then through a final die to produce wire of the desired diameter. Other patents teaching variations of this basic processing sequence are U.S. Pat. Nos. 3,826,690, 3,842,643, 3,852,875, 3,941,619, 3,952,571 and 3,962,898.
Wire drawing is typically facilitated by using specially formulated lubricants. Application of these materials to the wire permit it to pass easily through the drawing dies with a minimum of friction and stress. Typically, the lubricant is continuously applied to the wire prior to its passage through each individual drawing die, and is removed from the wire prior to its passage through the in-process anneal by complicated cleansing apparatus, such as shown in U.S. Pat. Nos. 3,826,690 and 4,280,857. Lubricant is removed because some of these materials burn excessively during the anneal, producing potentially hazardous and excessive amounts of smoke. Also, volatilization of the lubricant may create an oxide or carbonaceous residue on the wire surface, which must be removed prior to additional drawing.
During the in-process anneal, the length of the wire extends due to thermal expansion. Additionally, since the tensile strength of the wire is lower when it is at the anneal temperature than when it is at room temperature, if a tensile load is exerted on the wire during the annealing process, there may be additional wire elongation and possibly fracture of the wire. U.S. Pat. No. 2,932,502 shows one apparatus which compensates for any elongation of the wire which may take place during the anneal. This apparatus also minimizes the tensile forces which may be exerted on the wire while it is being annealed. Wire is wrapped several times around a rotating sheave before it passes through the anneal, and then is wrapped several times around another rotating sheave after it passes through the anneal. The two sheaves are connected to each other by differential gearing driven by an electric motor which independently adjusts the rotational speeds of the two sheaves, as required, to adjust the wire length and tension to the desired minimum level. Other patents showing means for maintaining wire tension are U.S. Pat. Nos. 3,328,554, 3,697,335, 3,826,690, and 4,280,857.
It is common in the fabrication of wire to use induction heating methods to anneal wire. In order to confine the induced current to the length of wire which is being annealed, two different methods have been devised to short out the electric current at the ends of this length, as shown in U.S. Pat. Nos. 3,328,554 and 3,826,690. In U.S. Pat. No. 3,826,690, a complicated arrangement of sheaves is utilized in order to obtain physical contact between the wire as it enters the annealer and exits the annealer. This contact short circuits the current which has been induced in the wire. In U.S. Pat. No. 3,328,554, the wire forms a loop inductively linked with an induction coil by passing over a first sheave, around a second sheave and back over the first sheave. The wire rides in one groove on the first sheave as it starts the loop, and in another groove on the first sheave as it completes the loop. While the sheaves are electrically nonconductive, the two grooves on the first sheave are electrically coupled to form a short circuited loop in which a current is induced which heats the wire. This patent also shows a similar short circuiting arrangement with two loops of wire connected in series which are inductively linked to an induction transformer.